. The reformation of the nuclear envelope after mitosis or after sperm nuclear entry into an egg is not well understood. It appears that the assembly reaction requires the binding of membrane vesicles to chromatin in an energy-independent, protein-dependent process. The fusion of these vesicles then occurs in an energy-requiring step which is followed by membrane growth. Lipophilic structures (LSs) in the nuclear envelope from a variety of sperm cell types have been identified. These structures correlate spatially with the location of sperm nuclear envelope fragments observed in fertilized eggs. The LSs are resistant to extraction by 0.1% Triton X-100 and lysolecithin and are stained with fluorescent lipophilic dyes. These LSs appear to be specialized regions of nuclear envelope involved in the targeted binding of a subset of membrane vesicles during the assembly of nuclear envelopes of sperm heads in egg cytoplasm. Vesicle binding appears to initiate in a region spreading from LSs and is dependent upon the presence of LSs. Upon addition of GTP to the binding reactions, the LSs fuse with the target vesicles. The specific aim of this proposal is to raise polyclonal and monoclonal antibodies to protein molecules present in the LSs of sperm nuclei. LSs will be partially purified and injected into rabbits or mice. Antigens recognized by the antibodies will be localized by immunofluorescence. Further experiments will center on the subset of antibodies which appear to stain appropriate structures. The antigens recognized by these antibodies will be analyzed further by immunoelectron microscopy, and the ability of the antibodies to inhibit nuclear envelope formation will be tested. The antibodies will be used to determine if LSs are present in somatic cells and antigens recognized by the antibodies will be microsequenced to identify the proteins and the genes cloned.